It is known to produce tool-hardened shaped structural parts for motor vehicle components, for example drive components like steering rods or cross bars, or structural components like door impact beams, B-columns, struts or shock absorbers which have material properties which are distributed uniformly over the shaped bodies. This is done by completely hardening the shaped body which can be in conjunction with an annealing or optionally a tempering process. These parts should have, on the one hand, a high strength so that they remain stable in, for example, a crash. On the other hand these parts should also be deformable in a crash so that the crash energy can be absorbed as deformation energy. In various applications in motor vehicle technology, shaped components should have a high strength over certain regions and other regions with a high ductility. For example, in the case of a B-column, the column foot should be relatively ductile, while high strength properties should be established on the upper part of the column.
Aside from reinforcement with additional plates or by joining together parts of different strength, it is already known to so treat a structural component by heat treatment that it has local regions of higher strength or higher ductility.
Thus DE 197 43 802 C2 describes a process for producing a shaped article for motor vehicle components with regions of different ductility and with a starting billet, before or after pressing is only partially heated or starting from a prior homogeneous heating is after heated in a targeted manner in the regions at which higher ductility is desired. The afterheating for producing ductile regions has however the drawback that the shaped body can distort.
DE 197 23 655 A1 describes a process for the partial hardening of a shaped body whereby a starting billet is homogeneously heated in a furnace and then hardened in a cooled pair of tools, whereby partial regions of the workpiece have hardening inhibited by slower cooling in that at these regions in the tool, recesses or thermally insulating inserts are disposed or in these regions in the tool the induction heating is applied. The purpose of this process is to provide nonhardened regions in the shaped body at which, additional machining, for example, drilling can be carried out. The method of DE 197 23 615 A1 is problematical in the context of a hot-forming process since at the locations of the recesses in the tool, shaping cannot occur and with larger ductile regions, thermally insulating inserts are provided in the tool which limit the hardening and interfere with the shaping process so that breakage is possible. The inductive hardening is possible only with finish-shaped parts and requires certain intrinsic operating steps. As a consequence the subsequent inductive hardening is expensive and has the danger of resulting in distortion.
European Patent EP 0 816 520 B1 describes a shaped article and a method for providing desired strength and hardening patterns over its length, whereby the shaped body, after its shaping, is inductively heated and then quenched to produce the hardened regions.
DE 200 14 361 U1 describes a B-column which also has regions of different strengths. The formation of the B-column is effected in a hot-forming process whereby starting from a blank or a preformed longitudinal profile, the workpiece is austenitized in a furnace and then shaped and hardened in a cooled tool. In the furnace large-area regions of the workpiece are insulated against the effect of the temperature whereby in these regions the austenitization temperature is not reached and as a consequence during the hardening no martensitic structure arises in the hardening and shaping tool.
Alternatively it has been proposed to initially completely austenitize the longitudinal section and in the transport in the hardening tool to limit the cooling in a targeted manner in a region so that it is not excessively rapid, for example by blowing on it to bring it to a temperature clearly below the austenitization temperature. In the hardening tool there may thus no purely martensitic structure formation but rather the formation of a mixed structure with clear ferrite/bainite components which has ductile properties.
This method has, when utilized practically in mass production, a number of problems. The insulation by encapsulation in a furnace is technologically expensive because in each cycle each individual part requires its own insulation, the application of the insulation must involve a previous preparation step for the heating process-and prolongs the latter and the insulation must be heated up in the case of repeated use. This makes the mass production system cost intensive. A targeted cooling down, which is not too precipitous for a limited region to a temperature significantly below the austenitization temperature during the transport process, is difficult to control because of the cooling conditions in mass production which makes it difficult to provide a corresponding temperature control for each product to be made.